Synthesis and characterization of sulfonated copolyethersulfones

Article abstract of DOI:10.1002/pola.24079

The present article deals with the synthesis and characterization of some sulfonated copolyethersulfones. The synthetic approach differs from the post sulfonation approach traditionally reported in the literature. The synthetic procedure is based on the u

Full text of DOI:10.1002/pola.24079

Synthesis and Characterization of Sulfonated Copolyethersulfones
FILIPPO SAMPERI,1 SALVATORE BATTIATO,1 CONCETTO PUGLISI,1 VINCENZO ASARISI,2 ANTONINO RECCA,2
GIANLUCA CICALA,2 RANIERO MENDICHI3
1Istituto di Chimica e Tecnologia di Polimeri (ICTP)-Sez. Catania, CNR, Via Gaifami 18, 95126 Catania, Italy
2Department of Chemistry and Physics Methodologies for Engineering (DMFCI), University of Catania,
Viale A. Doria 6, 95125 Catania, Italy
3Istituto per lo Studio delle Macromolecole (ISMAC) (CNR), Via E. Bassini 15, 20133 Milano, Italy
Received 17 February 2010; accepted 13 April 2010
DOI: 10.1002/pola.24079
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The present article deals with the synthesis and
characterization of some sulfonated copolyethersulfones. The
synthetic approach differs from the post sulfonation approach
traditionally reported in the literature. The synthetic procedure
is based on the use of sulfonated monomers which are then
reacted with previously synthesized telechelic hydoxy-ended poly
(ether sulpnone)s. Combining the MALDI-TOF MS and 1H NMR
analyses, with SEC-Viscometry and TGA measurements, we dem-
onstrate a powerful tool for characterizing the chemical composi-
tion, end chains, degree of sulfonation (DS) and molecular mass
distribution (MMD) of disulfonated poly(arylene ether-sulfone)
copolymers. The characterization techniques allowed to determine
the exact nature of the copolymers synthesized and to reveal
some interesting features about the reaction. DMA data show that
the glass transition temperature of sulfonated copolymers with
similar DS increase as raise their MMD. Copolymers with a DS of
ꢀ
C
V
10–11 mol % reach a Tg of 244–246 C. 2010 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 48: 3010–3023, 2010
KEYWORDS: copolymerization; functionalization of polymers;
gel permeation chromatography; MALDI; NMR; SEC-viscome-
try; sulfonated polyethersulfones; poly(ether sulfones)
INTRODUCTION Polyethersulfones are a class of polyaro-
matics of high importance for special applications which
includes toughening modifiers for epoxy resins,1,2 adhesives
for metal to metal bonding,3 membranes for separate gaseous
and solid substances4 and membranes for fuel cells.5,6 The lat-
ter application is of increasing interest for the industrial ex-
ploitation of fuel cells with improved properties. Ioioiu and
Sanchez7 recently outlined in a review the key advances of
polyethersulfones as membranes for proton exchange mem-
brane fuel cells (PEMFC) and solid oxide fuel cells (SOFC).
Sulfonated polyethersulfones constitute a promising class of
materials as alternative to perfluorinated polymeric mem-
branes because of their high mechanical strength and high
chemical, thermal, and oxidative stabilities.6,7 Several authors
have dealt with the problem of introducing sulfonic moiety
in the backbone of polyethersulfones. Among the different
synthetic routes, the most diffused are as follows:
The chemical modification of commercial polyethersulfones
either by aromatic electrophilic substitution10–12 or nucle-
ophilic substitutions13
;
The synthesis of sulfonated polymers starting from sulfonated
monomers or oligomers reacted with standard monomers.14–26
In the field of PEMFC, perfluorosulfonic acid ionomers such
R
R
as NafionV and FlemionV have made a great breakthrough.
These perfluorinated polymeric membranes provide high
conductivities, up to 80 ꢀC, thanks to their superacid per-
fluorosulfonic moieties and the high thermal and chemical
stabilities related to their perfluorinated backbones.8 Despite
their advantages, existing drawbacks, namely high cost, high
permeability to methanol, low conductivities and poor me-
chanical properties at temperatures exceeding 80 ꢀC, make
their use questionable in the context of PEMFC or direct
methanol fuel cells (DMFC). Great efforts have been devoted
toward the development of high-performance proton-con-
ducting polymers for the mass market which can offset the
limits of perfluorinated polymeric membranes.5,9
The first approach is cheapest but can lead to chain break-
ings during industrial production.27 Furthermore, the use of
commercial polymers can encounter the additional problem
that molecular mass distribution, and in general viscosity
properties, are not optimized for thin film preparation
required for membrane generation. The second approach
based on the synthesis from monomers present the advant-
age of close control on the final structure of the backbone
and of the properties (i.e., molar mass distribution, chemical
structure, end groups etc.) of the polymers. An additional
advantage of the synthesis from oligomers is the possibility
to obtain controlled multiblock structures which lead to the
Correspondence to: F. Samperi (E-mail: fsamperi@unict.it)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3010–3023 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
formation of well separated domains between the hydropho-
bic blocks (i.e., nonsulfonated) and the hydrophilic blocks
(i.e., sulfonated). The creation of such morphologies has
been outlined as a key advantage to control the water swel-
ling properties of the membranes.7
Therefore, in this work we have focused our studies to dem-
onstrating the enormous potentiality of combining MALDI
and NMR techniques for a complete and unequivocal chemi-
cal characterization of sulfonated PES copolymers, to moni-
tor their degree of sulfonation and also the progress and the
kinetic of their polymerization reactions. Furthermore, in the
present article the study of the copolymerization has been
advanced by the use of SEC-viscometry which allows the
careful determination of the molar mass distribution upon
polymerization. Finally, the polymers obtained have been
characterized in terms of their thermal properties by TGA
and DMA. The control of the copolymerization conditions
and the knowledge of: the nature of chain ends, the chemical
composition and of the degree of sulfonation, may lead to
samples with better properties than the sulfonated PES al-
ready synthesized and studied, which, therefore, could be
used as polyelectrolyte proton exchange membranes in sev-
eral fields, that is, as membranes for fuel cells.
The exact knowledge of the chemical structure of the back-
bone of polymers is of paramount importance when novel
polymers are developed to evaluate their performances and
potential application. Among the various techniques which
can be used for this purpose NMR and MALDI-TOF MS (Ma-
trix-Assisted Laser Desorption/Ionization–Time of Flight
Mass Spectrometry) are among the most powerful tools.
Recently, the combined use of NMR and MALDI-TOF MS has
been reported for the fine characterization of some reactive
copolyethersulfones.28 The combined use of the two techni-
ques has revealed high potentiality to shed more light on the
structure and chemical composition of complex copolymers
and to disclose the nature of side reactions.
In the present article, we present the synthesis of some sul-
fonated copolymers based on the use of sulfonated mono-
mers reacted with some telechelic PES oligomers terminated
with phenol groups at both ends (hereafter referred to as
PES-OH) and having a number average molecular mass (Mn)
of 4000 and 7000 g/mol, respectively. The nucleophilic aro-
matic polycondensation reactions were carried out for differ-
ent times using different molar ratio of PES-OH samples and
disulfonated dichlorodiphenyl sulfone monomer (referred as
SDCDPS). The sulfonated PES copolymers synthesized are thor-
oughly characterized by NMR and MALDI-TOF MS to reveal
their chemical structure, while thermogravimetric (TGA) and
dynamic mechanical analysis (DMA) are used to determine the
thermal properties of the polymer samples. Infrared spectros-
copy (IR) was also used for the characterization of the sulfonic
moiety in the main backbone of the polymers. Their molar
mass distribution (MMD) was investigated by a multidetectors
size exclusion chromatography (SEC) system.
EXPERIMENTAL
Materials
4,40-Dichlorodiphenylsulfone (DCDPS) (98% of purity grade),
dihydroxydiphenylsulfone (DHDPS) (98% of purity grade)
hydroquinone (HQ) (99% of purity grade), potassium car-
bonate, and tetramethylenesulfone (sulfolane) were pur-
chased from Aldrich Chemical (Italy). All reagents used in
these experiments were obtained from commercial sources
and utilized without any special purification. Potassium car-
ꢀ
bonate [K2CO3, anhydrous grade] was dried at 150 C under
reduced pressure. Tetramethylenesulfone was distilled under
reduced pressure and stored under nitrogen until used.
Synthesis
Hydroxy-Ended PES Prepolymers
Two hydroxy-ended poly(arylene ether-sulfone)s with an av-
erage molar mass of 4000 and 7000 g/mol, respectively,
were synthesized via nucleophilic condensation reaction of
DCDPS with an excess of DHDPS as is outlined in Scheme 1.
Typically, the polycondensation reaction was conducted in a
250-mL-three-neck flask equipped with an overhead stirrer
and nitrogen inlet. To obtain the hydroxy-ended PES prepoly-
mer having a theoretical average number molar mass (Mn)
of 4 K [herein referred as PES-OH(4 K)], DCDPS (5.11 g, 1.78
ꢁ 10ꢂ2 mole) was added at DHDPS (4.89 g, 1.95 ꢁ 10ꢂ2
mole; 9.5 mol % excess) in 50 mL of tetramethylenesulfone
solvent. K2CO3 (2.53 g, 1.83 ꢁ 10ꢂ2 mole; 2.8% mole excess
with reꢀspect to the DCDPS) was then added to the mixture
The chemical composition of S-PESs were generally charac-
terized by NMR and FTIR techniques.16–18,20–23,29–35 How-
ever, these are averaging techniques and often some doubts
may remain on the composition of the sulfonated PES sam-
ples, and on the nature of the end chains, which can influ-
ence the properties of the based polyelectrolyte membranes.
In particular, often by these techniques is not easy to distin-
guish block copolymer from the corresponding homopoly-
mers present in the polymeric material. When the Mn of the
polymer sample is very high, the end groups could not be
detected by NMR techniques.
ꢀ
at 180 C and the temperature was raised up to 190 C. The
mixture was allowedꢀto react for 30 min atꢀthis temperature,
then for 1 h at 205 C and for 4 h at 225 C, under a nitro-
gen flow. The reacted mixture was then added at a cooled
ethanol (5- to 10-fold in excess), obtaining a precipitated
white powder, which was filtered and then washed several
times with water and with ethanol, and finally dried under
vacuum at 120 ꢀC overnight. The sample was then treated
with THF at reflux for 12 h to solubilize the PES oligomers
with a molar mass below 1500–2000 g/mol, as verified by
MALDI-TOF analysis of the THF soluble fractions.
MALDI-TOF MS is a powerful technique able to detect indi-
vidual and intact polymer molecules even in a complex mix-
ture, including species present in smaller amounts in a poly-
mer sample.36–40 It has permitted the identification of repeat
units, chain ends, cyclic oligomers, and also of species pres-
ent in a smaller amount in a lot of class of polymer samples.
To the best of our knowledge, no data has ever been
reported by literature on the characterization of sulfonated
PES samples by MALDI-TOF MS technique.
SULFONATED COPOLYETHERSULFONES, SAMPERI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of tele-
chelic PES-OH.
A PES-OH sample with a Mn of 7K g/mol [PES-OH(7 K)] has
been synthesized using a DHDPS/DCDPS molar feed ratio of
1.05, following the same procedure.
ophilic condensation reaction of sodium salt disulfonated
monomer with an telechelic PES-OH sample, using various
PES-OH/SDCDPS molar feed ratio at 180 ꢀC for 12 h. The
amount of DS-DSDCPS was calculated with respect to the con-
centration of hydroxyl end groups in the used PES-OH. In par-
ticular, four molar feed ratio (1.20, 1.05, 1.00, and 0.90) were
used to prepare S-PES copolymers with different degrees of
sulfonation (DS) and molar mass distributions (MMD), as
described in Table 1. The synthesis is outlined in Scheme 3.
3,30-Synthesis of Disulfonated-4,40-dichlorodiphenyl
Sulfone (SDCDPS)
Disodium of disulfonated dichlorodiphenyl sulfone momomer
(DS-DSDCPS) was synthesized using 4,40-dichlorodiphenyl
sulfone (DCDPS) and fuming sulfuric acid, via electrophilic
aromatic substitution, as reported in Scheme 2.41 The poly-
merization was carried out into a 100-mL-three-necked flask
equipped with a mechanical stirrer and a nitrogen inlet/outlet.
DCDPS (28.7 g) was dissolved in 60 mL of fuming sulfuric acid,
and the solution was heated to 110 ꢀC for 6 h. The reaction mix-
ture was allowed to cool to room temperature and added into
400 mL of ice water. Next, 180 g of NaCl was added and, subse-
quently, the disodium salt of disulfonated dichlorodiphenyl sul-
fone precipitated as a white powder. The latter was filtered and
redissolved in 400 mL of cold deionized water, treated by 2 N
NaOH aqueous solution to a pH of 6–7, and finally an excess
amount of NaCl was added again to obtain the sodium form of
the sulfonated monomer. The crude product was filtered and
recrystallized from a mixture of alcohol (methanol) and deion-
ized water (9/1, v/v), producing white needle-like crystals. The
DS-SDCDPS monomer was analyzed by MALDI-TOF MS using
dithranol as a matrix), and by (1H, 13C) NMR tools. The results
confirmed its structure and composition. MALDI-TOF mass
spectra, besides peaks due to the matrix, shows a signal at m/z
490.3 6 0.2 corresponding to the molecular mass of DS-
SDCDPS sample. 1H NMR spectrum presents signals at d (ppm):
7.6 (d, 1H); 7.8 (m; 1H); 8.3 (d, 1H), while 13CNMR spectrum
shows six expected signals, corresponding to the aromatic car-
bons, at d (ppm): 128, 130, 133, 138, 139, and 147.
Typically, polycondensation was carried out in a 250-mL-
three-neck flask equipped with an overhead stirrer and
nitrogen inlet using tetramethylenesulfone as solvent and
toluene as azeotropic agent. The reaction mixture was
ꢀ
heated at 140 C for 3 h, which stripped of most the toluene
to dehydrate the system. The temperature was raised to
180 ꢀC and the reaction was carried out for (12 h) hours
under a nitrogen flow. The reacted mixture was then precipi-
tated in 10-fold volume of a HCl aqueous solution 1 M; the
precipitate white powder copolymer was washed several
times with water and ethanol, and finally dried under vac-
uum at 120 ꢀC overnight. As well as the PES-OH prepoly-
mers, all copolymers were treated with THF at reflux to
extract the low molar mass oligomers. After that were fil-
tered and finally dried under vacuum at 120 ꢀC overnight.
The copolymer samples were referred as S-PES-mr(12 h) in
Table 1, where with mr we indicate the PES-OH/SDCDPS
molar feed ratio.
Postpolymerization of S-PES-mr(12 h)
The postpolymerization reaction of sulfonated poly(arylene
ether-sulfone) (S-PES) obtained after 12-h reaction, has been
ꢀ
carried out at 180 C under nitrogen flow in NMP for further
24 and 48 h, in presence of K2CO3 (0.01 g), under nitrogen
flow. The product of reaction was precipitated in ethanol
and washed several timesꢀ in the same solvent. Finally, the
product was dried at 150 C overnight. The samples namely
Sulfonated Poly(arylene ether-sulfone) Copolymers
Di-sulfonated poly(arylene ether-sulfone)s (S-PES), having
blocks of unsulfonated PES units were synthesized via nucle-
SCHEME 2 Synthesis of monomer sulfonated dichloro diphenil-sulfone sodium salt.
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DSd (%)
TABLE 1 Some Properties of Sulfonated and Unsulfonated PES Samples
Samples
Mn
Tg (ꢀC)b
Char residuec (%)
ginh.
a
PES-OH (4 K)
4000
160
210
238
244
200
241
246
182
266
262
30
0.08
0.095
0.11
0.13
0.09
0.12
0.14
0.19
0.24
0.21
0
S-PES-0.90 (12 h)
S-PES- 0.90(36 h)e
S-PES-0.90(60 h)e
S-PES-1.20(12 h)
S-PES-1.20(36 h)e
S-PES-1.20(60 h)e
PES-OH (7 K)
8000
40.08
40.85
41.44
37.37
40.65
41.10
30
11
11
11
10
10
10
0
13,100
16,800
7000
13,800
15,700
7000
S-PES-1.00(12 h)
S-PES-1.05(12 h)
14,000
13,100
42.19
43.01
14
12
a
d
e
Calculated by 1H NMR analysis.
Degree of sulphonation.
b
Calculated by DMA measurement.
Determined by TGA analysis at 800 ꢀC.
Synthesized by postcondensation reaction of S-PES-12 h samples.
c
as S-PES-mr(12 þ x h)(with x ¼ 24 or 48) are reported in
NMR software applying the Lorentz-Gauss enhance function
using appropriate Line broadening and Gaussian broadening
parameters to improve the peaks resolution. The chemical
shift resonances were accurately assigned to the specific pro-
ton and atoms, using the data reported in literature.28,42
Table 1.
Measurements
NMR Spectroscopy
1H NMR spectra were recorded using a Brucker 200 MHz.
The samples were dissolved in d6-DMSO at polymer concen-
tration of about 30 mg/mL without any internal standard at
MALDI-TOF Mass Spectrometry
The MALDI-TOF mass spectra were recorded in linear mode,
using a Voyager-DE STR instrument (Perseptive Biosystem)
ꢀ
60 C in temperature. The data were elaborated with 1D Win-
SCHEME 3 Typical synthesis of sulfonated poly(arylene ether-sulfone)s.
SULFONATED COPOLYETHERSULFONES, SAMPERI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
mass spectrometer, equipped with a nitrogen laser (k ¼
337 nm, pulse width ¼ 3 ns), working in a positive ion
mode. The accelerating voltage was 25 kV, grid voltage and
delay time (delayed extraction, time lag), were optimized for
each sample to achieve the higher mass resolution (FWHM).
Laser irradiance was maintained slightly above threshold.
Samples used for the MALDI analyses were prepared as fol-
lows. Ten microliters of polymer solutions (3–4 mg/mL in
CHCl3/DMSO 90/10), were mixed with 30 lL of HABA solu-
tion (0.1 M in CHCl3/THF 90/10 v/v). Then 1 lL of each
analyte/matrix mixture prepared was spotted on the MALDI
sample holder and slowly dried to allow analyte/matrix
cocrystallization.
FTIR Spectrometry
FTIR spectra of sulfonated poly(arylene ether-sulfone) in
powder form, has been recorded on attenuated total reflec-
tance spectroscopy (ATRIR) using a Perkin–Elmer FTIR spec-
trometer. Spectrometric analysis is useful to obtain further
information regarding chemical structure and degree of sul-
fonation of PES samples studied because of the typical peaks
of each functional group.
Dynamic Mechanical Analysis
Viscoelastic properties of each sample were determined by
dynamic mechanical analysis (DMA). For the DMA analysis
the specimens were prepared according to the following pro-
cedure: The polymers obtained from the syntheses were
finely micronized in powder with an average dimension of
SEC-Viscometry
ꢀ
30 mm; and vacuum dried at 80 8 C for 24 h. Then, 0.35 g
The molecular characterization (molar mass distribution
MMD, intrinsic viscosity and conformation) of PES-OH pre-
polymers and of S-PES copolymers was performed by a mul-
tidetector size exclusion chromatography (SEC) system using
N,N-dimethylacetamide (DMAc) as mobile phase. The chro-
matographic system consisted of an integrated GPCV2000
chromatographic system from Waters (Milford, MA) using
two on-line detectors: (1) differential viscometer (DV); (2)
differential refractometer (DRI model 2414) used as concen-
tration detector. This multidetector SEC-DV system has been
described in detail elsewhere.43 The column set was com-
posed of two Styragel mixed columns (HR5E and HR4E,
5 lm of particle size) from Waters. The experimental condi-
tions consisted of: DMAc þ 0.01 M LiBr as mobile phase,
80 ꢀC of temperature, 0.8 mL/min of flow rate. It is well
known that the on-line DV detector furnishes the relative vis-
cosity gr of each eluting polymeric fraction. By combining the
instantaneous values of gr and of the concentration, from the
on-line DRI calibrated detector, the [g] ¼ ƒ(V) experimental
function, where [g] denotes the intrinsic viscosity and V the
elution volume, is directly obtained. From the SEC-DV system
also some conformational properties of the polymers could be
studied by means of the Mark-Houwink-Sakurada (MHS) equa-
tion [g] ¼ ƒ(M) where M denotes the molar mass.
of polymer powder was weighted in a standard material
pocket purchased from Triton and pressed in a Carver press
to obtain a uniform film within the pocket. The pocket was
tested in single cantilever mode at 1 Hz with a free length of
15 mm. The samples were heated according tꢀo the following
ing under nitrogen flow to 0 ꢀC; (3) heating at 5 ꢀC/min
from 0 to 280 ꢀC. The glass transition was calculated in the
second heating scan.
ꢀ
cycle: (1) heating at 5 C/min from 0 to 280 C; (2) quench-
Off-Line Viscosimetry
Inherent viscosity (ginh ¼ ln grel/C; C ¼ 0.5 g/dL) was also
determined by off-line viscosimetry using an Ubbelohde sus-
pended-level viscometer in dimethylformamide (DMF) as sol-
ꢀ
vent at 25 C.
RESULTS AND DISCUSSION
Two series of sulfonated copolyether sulfones (S-PES) with
controlled degree of sulfonation and having unsulfonated
PES blocks were successfully synthesized through nucleo-
philic aromatic substitution polycondensation reaction of so-
dium salt of sulfonated dichloro difenilsulfone (S-DCDPS)
with hydroxyl-ended PES telechelic polymers (PES-OH), as
outlined in Scheme 3. The copolymer synthesis involves con-
trolled amount of sulfonated activated halide (S-DCDPS) and
a PES-OH sample with a note average molar mass (Mn) of
4000 or 7000 g/mol. Polycondensation reactions were car-
ried at 180 ꢀC for 12 h using various PES-OH/S-DCDPS
molar ratios, calculated with respect to the hydroxyl amount
of telechelic PES-OH prepolymers, taking into account its Mn
values (see Table 1) and also that two OH groups are pres-
ent in each chain. The synthesized copolymers are herein
referred as S-PES-mr(x h), where mr represents the (PES-
OH)/S-DCDPS molar feed ratio and x h the reaction time.
The chemical compositions of all copolymers were evaluated
by 1H NMR, FTIR, and MALDI-TOF MS analyses, whereas
their thermal properties and thermal stability were investi-
gated by TGA and DMA analyses. By 1H NMR analysis were
also determined the degree of sulfonation (DS) and the Mn
values of the sulfonated copolymers, while by viscosimetric
measurements we have obtained some information about
their molar masses (MM). Reliable values about their molar
mass distribution were also calculated by SEC-DV tool.
Thermogravimetric Analysis
Thermal stability of sulfonated poly(arylene ether-sulfone)s
was studied using a TGA-500 V6.7 instrument (TA-INSTRU-
MENT) coupled with a TA-INSTRUMENT Explorer operating
software. The analyses were performed under dynamic heat-
ing conditions, from 50 to 800 ꢀC under nitrogen flow
(60 mL/min), at a heating rate of 10 ꢀC/min, using about
2 mg of sample. The weight and temperature of the thermo-
balance were calibrated using four high-purity magnetic
standards (alumel, nickel, Ni0.83Co0.17 alloy, Ni0.63Co0.37 alloy)
ꢀ
for the Curie temperature (152.6, 358.2, 554.4, and 746.4 C,
respectively) and some exact weight following both calibra-
tion procedures through the instrument control software.
Data recorded show the thermal behavior of poly(arylene
ether-sulfone) samples in terms of weight loss percentage at
the increasing of temperature. The slope of degradation
curve depends on kind and amount of specific mechanism of
degradation during temperature scanning.
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FIGURE 1 MALDI-TOF mass spectrum of PES-OH(4 K).
The S-PES copolymers were analyzed in detail by MALDI-TOF
MS to identify how many disulfonated PES units are present
for each chain and the various types of end groups, and also
to afford a deeper insight in the reaction mechanism.
and b00, respectively. The aromatic protons in ortho to the sul-
fone groups of diphenyl sulfone moieties [protons b in Fig.
2(a) and in Table 3] and in ortho to the oxygen of diphenyl
ether groups [protons a in Fig. 2(a) and in Table 3] in the
backbone of PES-OH chains, give two doublets at 7.98 and
7.28 ppm, respectively. The Mn of both PES-OH polymers were
First, two PES-OH samples with an average number molar
mass (Mn) of 4000 and 7000 g/mol, [herein referred as PES-
OH(4 K) and PES-OH(7 K), respectively] were synthesized
using a DHDPS/DCDPS molar ratio of 1.09/1 and 1.05/1,
respectively, as outlined in Scheme 1. The nature of end
groups of both PES-OH samples was confirmed by MALDI-
TOF MS and 1H NMR studies. MALDI-TOF mass spectrum of
prepolymer PES-OH(4 K) is shown in Figure 1, it shows a
single mass peak family, in the mass range 1000–8000 Da,
corresponding to the sodiated ions of expected PES chains
terminated with hydroxyl groups at both ends (species Dn in
Fig. 1). The mass spectrum of PES-OH(7 K), as well as of
PES-OH(4 K) in Figure 1, shows only single intense mass
peak series corresponding to the sodiated ions of dihydroxy
ended PES chains with n (repeating units) ranging from 9
(m/z ¼ 2539) to 46 (m/z ¼ 11,944); it shows the most
intense peak at m/z 7469 corresponding to the oligomers
with n ¼ 28. 1H NMR spectra of both PES-OH samples con-
firms this result, in fact they show two doublet centered at
6.82 ppm and at 7.67 ppm due to the resonance of the aro-
matic protons in ortho and in meta with respect to the
hydroxyl end groups, as can be observe in Figure 2(a) that
reports the spectrum of PES-OH(4 K). In Figure 2(a), the sig-
nals due to the resonances of these protons were labeled as a00
1
calculated from the relative H NMR integrals of the peaks due
00
to the aromatic protons Hb belong to the end chains and the
aromatic resonances due to the protons Ha in the backbone,
using the eq 1. The calculated values are reported in Table 1.
00
Xn ¼ ðIb =IaÞ þ 1
(1)
Using both PES-OH samples we have synthesized four sulfo-
nated PES copolymers, namely S-PES-0.5(12 h), S-PES-0.9
(12 h), S-PES-1.0(12 h) and S-PES-1.2(12 h).
These sulfonated PES copolymers were well characterized by
MALDI-TOF MS and by 1H NMR tools to obtain reliable informa-
tion about their chemical composition and, especially on the na-
ture of the end groups and also to know many sulfonated units
are present along the chains.
MALDI-TOF MS
The MALDI-TOF mass spectrum of S-PES-0.9(12 h) sample is
shown in Figure 3. It shows complex clusters of peaks from
m/z 1300 up to m/z 9600, the mass difference between two
homologous peaks belonging two contiguous clusters is
SULFONATED COPOLYETHERSULFONES, SAMPERI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
with OH groups at one chains and chlorine at other end
(species Bn,a¼2) dominate in the spectra of the S-PES-
1.2(12 h) copolymer. On the basis of the nature of end
chains, in the co-oligomer chains belong to the species Bn,a¼2
one disolfonated unit is present as end chain. Cycles co-
oligomers (species Un,a) having two d-S-PES units are also
observed in the mass range m/z 1300/4000 of the mass
spectra of both copolymers. According to the expanded spec-
tra in Figure 2(b,c) we argue that the formation of cycles, if
favored when the PES-OH/S-DCDPS molar feed ratio is more
1.0. In the spectra of both samples we observe also the pres-
ence of co-oligomers having three d-S-PES units (Xn,a¼3
)
along the chains, which one d-S-PES unit appear as mono so-
dium salt. The presence of these family of peaks indicates the
incomplete conversion of sodium form synthesized S-PES
copolymers in the corresponding acid form, by treatment with
a solution of HCl 1 M (see Experimental section). Mass spectra
of both copolymers, in the mass range higher than m/z 3400,
show peaks corresponding to co-oligomers with three d-S-PES
units belonging to the families An,a and Bn,a. The same families
of co-oligomers (An,a, Bn,a, Un,a, and Xn,a) were also observed in
the MALDI-TOF mass spectra of disulfonated copolymers pre-
pared from PES-OH(7 K) and referred as S-PES0.5(12 h) and
S-PES0.5(12 h) in Table 1. MALDI-TOF mass spectrum of
S-PES1.05(12 h), as well that of S-PES1.0 (12 h), shows homol-
ogous cluster of peaks from m/z 1500 up to m/z 10,000 and
the most intense peaks are around m/z 5000 Da [Fig. 4(a)].
Comparing this mass spectrum with that of the S-PES0.9(12 h)
in Figure 3(a), emerge that the copolymers obtained from PES-
OH(7 K), by 12-h reaction, have an average molar masses (Mw
and Mn) higher than of those synthesized from PES-OH(4 K),
as confirm the viscosimetric and SEC-DV data in Tables 1 and
4, respectively. By inspection the enlarged sections of mass
spectra of S-PES1.0(12 h) and S-PES0.5(12 h), in the mass
range m/z 4150–4850, reported in Figure 4(b,c), respectively,
we observe that these copolymers, beside the co-oligomer spe-
cies discussed before, have also chains terminated with Chlo-
rine at both ends (specie Cn,a, in Fig. 4 and in Table 2).
FIGURE 2 1H NMR spectra of PES-OH(4 K) (a); S-PES-1.2(12 h)
(b); S-PES-1.05(12 h) (c); S-PES-1.0(12 h) (d); S-PES-0.9(12 h) (e).
NMR Characterization
1H NMR spectra were also used to identify the molecular
structure, the chemical composition, the end chains, and also
to calculate the degree of sulfonation of the four S-PES
232 Da (the mass of no-sulfonated PES repeating unit). The
mass spectrum of S-PES-1.2(12 h) is very similar. Detailed in-
formation about the chemical composition and end chains of
these copolymers can be argued from the expanded mass
regions of their spectra, from m/z 2300 up to m/z 3000,
reported in the inset in Figure 3 [Fig. 3(b,c)]. Both spectra
show that each cluster present in the mass spectrum, is con-
stituted of four families of peaks corresponding to the copol-
1
copolymers discussed above. Complete assignments of the H
NMR spectra have been accomplished by comparison with lit-
erature data,28,42 and with the spectra either of PES-OH(4 K)
[see Fig. 2(a)] and of SDCPS monomer. The chemical shift
resonances are summarized in Table 3. The 1H NMR spectra
of the disulfonated PES copolymers obtained after 12 h reac-
tions and that of PES-OH(4 K) are overlaid in Figure 2.
ymer chains with different end groups (species An,a, Bn,a
,
U
n,a, and Xn,a); their pertinent assignments are summarized
The Figure 2 reports also the assignments of all peaks accord-
ing to the chemical shifts assignments reported in Table 3. It
permits to distinguish the disulfonated PES units from those
unsulfonated PES blocks. These give two doublets centered at
7.98 ppm (protons Ha) and at 7.27 ppm (protons Hb), whereas
the disulfonated PES units give signals in the regions: 8.35–
in Table 2 where with n we indicate the unsulfonated PES
units and a the disulfonated PES units. For SPES-0.9(12 h)
sample the most intense mass peaks correspond to the co-
oligomers with 2 disulfonated PES units (d-S-PES) along the
chains and terminated with hydroxyl groups at both chains
(Species An,a¼2), while the peaks corresponding to the fami-
lies of co-oligomers with two d-S-PES units and terminated
0
8.41 ppm (protons H3 and H3 ), 7.84–7.91 ppm (protons H2 and
0
0
H2 ), 7.50–7.62 ppm (protons H1 and H2 ) and 7.12–7.20 ppm
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ARTICLE
FIGURE 3 MALDI-TOF mass spectra of S-PES-0.9(12 h) (a) and enlarged section of the mass spectra of S-PES-0.9(12 h) (b) and
S-PES-1.2(12 h) (c).
(protons H1). The sufficient peak resolution permits to distin-
guish also the S-PES units in the backbone from those in the end
chains; that is, the peaks H3 belong the disulfonated units in the
main chains that link two unsolfonated PES blocks, while the
From 1H NMR spectra we can also calculate the mol % of
sulfonated units (S-PES) present as end chains (CS-PES-ends
with respect to total amount of S-PES along the macromolec-
ular chains, by eq 7
)
0
peaks H3 belong to same units in the end chains. From proton
spectra in Figure 2(b–d) we can also characterize the phenol
CS-PES-ends ¼ IH ðIH þ IH 0 Þ ꢁ 100
(7)
0
3
3
3
00
00
end groups (peaks Ha and Hb ). The intensities of the peaks
due to the end groups decrease as increase the Mn of S-PES
copolymers obtained after 12 h reactions. Since, using the inte-
gral area (I) of protons belonging to the unsolfonated PES
blocks (protons Ha) and those due to the S-PES units (protons
MALDI-TOF MS and 1H NMR spectra revealed that all S-
PES(12 h) copolymers are terminated with phenol and chlo-
rine groups. This evidence suggest that nucleophilic substitu-
tion of both chlorine belonging to the sulfonated monomer
used for synthesized them is not complete, and therefore it
acts as coupling agent when both chlorine atoms are substi-
tuted and as terminal when only one chlorine atom is substi-
tuted during the polycondensation reaction between PES-OH
and DS-DSDCPS. These data suggest also the kinetic of the
substitution of chlorine atoms belong to the DS- DSDCPS is
may be lower with respect to the corresponding unsulfo-
nated DCDPS monomer. On the base of this result, we
thought that the nucleophilic substitution condensation reac-
tion between two copolymer chains terminated with these
groups could be carried further giving a chain coupling, and
thus an increase in molar masses distributions (MMD) of
each copolymer. To confirm our hypothesis, we performed
the postcondensation reaction of the S-PES0.9(12 h) and
0
H3 and H3 ), we have calculated the molar fractions of sulfo-
nated (referred as CS-PES) and unsolfonated PES units (CPES), the
degree of sulfonation (DS), the degree of polymerization (Xn)
and the Mn of the S-PES(12 h) copolymers, using the eqs 2–6.
CPES ¼ ðIH =4Þ=ðIH =4 þ IH þ IH 0 Þ
(2)
(3)
(4)
(5)
(6)
a
a
3
3
CS-PES ¼ 1 ꢂ CPES
DS ¼ 2 ꢃ CS-PES
Xn ¼ ðIH =4 þ IH =2Þ=ðCPES ꢃ IH =2 þ CS-PES ꢃ IH 0 Þ
00
a
3
a
3
Mn ¼ Xn ꢃ ð232:2 ꢃ CPES þ 376:3 ꢃ CS-PES
Þ
where 232.2 and 376.3 correspond to the mass of unsolfo-
nated as sulfonated repeating units, respectively.
ꢀ
The DS values, which define the average number of sulfonic
acid groups (ASO3H) in S-PES copolymer chains, correspond
to the double of CS-PES molar fractions (DS ¼ 2 CS-PES), as
each sulfonated unit presents two SO3H groups.
S-PES1.2(12 h) samples for other 24 and 48 h at 200 C, as
described in the experimental section. The progress of the
postcondensation reaction has been followed by 1H NMR
and MALDI-TOF MS analyses. The MALDI-TOF MS results
SULFONATED COPOLYETHERSULFONES, SAMPERI ET AL.
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ARTICLE
FIGURE 4 MALDI-TOF mass spectrum of S-PES-1.05(12 h) (a). The inset reports the enlarged sections of the spectra of samples S-
PES-1.0(12 h) (b) and S-PES-1.05(12 h) (c).
indicate that the copolymers chains terminated with chlorine
groups decrease with respect to those ended with phenol
groups at both ends, confirming the progress of the conden-
sation reaction. MALDI TOF mass spectra (not reported here
for brevity) show homogeneous cluster of peaks ranging
from m/z 2000 to m/z 26,000, indicating that the MMD of
the copolymers increase, as expected. In these mass spectra
the most intense peaks correspond to the families termi-
nated with OH groups at both ends (species An,a with a ¼ 2
in Table 2), intense peaks corresponding to the same family of
copolymer chains with a ¼ 3 and a ¼ 4 were also observed
in the mass range higher m/z 4000. The intensities of peak
series Bn,a and Cn,a is very lower than of those observed
in mass spectra of copolymer samples obtained after 12 h
reactions (Fig. 3). Their intensity decrease compared to those
of mass peak families An,a with increasing reaction time.
Comparing the MALDI-TOF mass spectra of the copolymers
obtained by postcondensation reaction with respect to
those of the copolymers obtained at 12 h, in the mass range
m/z 2000–5000 we have also observed un increment of
the relative intensity of cycles co-oligomers (species Un,a in
Table 2). These data suggest that cycles could be formed by
postcondensation reactions involving the OH and Cl end
groups belonging to the same macromolecular chain (species
B in Table 2) or to two different chains (i.e., species Bn,a
and Cn,a in Table 2). The kinetic of the postcondensation
reaction was followed by means of 1H NMR spectra of the
formed copolymers. As an example spectra of S-PES0.9(12 h),
S-PES0.9(36 h) and S-PES0.9(60 h) are overlaid in Figure 5.
With the other peaks normalized, it can be observed that as
the postreaction time increase, the intensity of signals due to
0
aromatic protons belonging to the end groups (protons: H3
00
00
at 8.38 ppm, Hb at 7.68 ppm, Ha at 7.69 ppm; see Table 3
for their assignments) gradually become smaller, indicating
that the molar mass distributions increase, as confirm the
1
Mn values calculated from the H NMR spectra, which values
are gathered in Table 1. These results were also confirmed
by SEC-Viscometry and off-line viscometry measurements
(see data in Table 1 and ‘‘Discussion’’ section). The degree of
sulfonation of S-PES copolymers prepared by postcondensa-
tion reaction was calculated from their 1H NMR spectra. For
each copolymer the DS value does not change with respect
to that of the corresponding S-PES(12 h) parent sample.
SEC-Viscometry
The summary of the molecular characterization of the PES-
OH(4 K) and four S-PES samples by SEC-Viscometry is
shown in Table 4. The table reports the molar mass of the
chromatographic peak (Mp), the more important molar mass
averages (Mn, Mw, Mz), the polydispersity indexes (Mw/Mn,
Mz/Mw), the intrinsic viscosity [g] in DMAc þ 0.05 M LiBr
solvent at 80 ꢀC of temperature and finally the coefficients
of the MHS plot (intercept k and slope a). Wang et al.44 have
revealed that intrinsic viscosity data for sulfonated PES
SULFONATED COPOLYETHERSULFONES, SAMPERI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
copolymers were not comparable to those of nonsulfonated
polymers, since the polymer electrolyte chains interact via
sulfonate groups. To suppress the polyelectrolyte effects, in
accord with some authors23 we have used NMP with 0.05 M
LiBr, because the small amount of salt effectively suppressed
the polyelectrolyte effect allowing improved characterization
of the ion containing materials.
Data reported in Table 4 clearly confirm previous conclu-
sions on the S-PES polymerization. By increasing the time of
polymerization from PES-OH to S-PES 12 h and S-PES 60 h
the average molar mass evidently increases. Specifically for
the S-PES0.9 samples the weight average Mw increases from
8.1 kg/mol (PES-OH) to 21.4 kg/mol [S-PES 0.9(60 h)]. Quite
interesting the polydispersity of the samples was narrow
and almost constant ranging from 1.3 to 1.4. This polymer-
ization tendency is evident also for the S-PES1.2 samples.
Figure 6 shows the comparison of the differential MMD of
four samples: PES-OH(4 K), S-PES 0.9(12 h), S-PES 0.9(36 h)
and S-PES 0.9(60 h). MMDs of samples clearly show the tend-
ency in molar mass increasing. The molar mass increases
from PES-OH to PES 0.9(12 h) and furthermore increases to
PES 0.9(36 h) but there is not meaningful difference in molar
mass between PES 0.9(36 h) and PES 0.9(60 h) samples.
Figure 7 shows the comparison of the MHS plot, [g] ¼ k Ma,
of four samples: PES-OH(4 K), S-PES 0.9(12 h), S-PES 0.9
FIGURE 5 1H NMR spectra of S-PES-0.9 samples obtained at
different reaction time: 12 h (a), 36 h (b), and 60 h (c).
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TABLE 4 Summary of the Molecular Characterization of PES-OH and S-PES Samples by SEC-DV
Sample
Mp g/mol
Mn g/mol
Mw g/mol
Mz g/mol
Mw/Mn
Mz/Mw
[g] dL/g
K
a
PES-OH(4 K)
7,300
6,300
8,100
10,700
17,300
25,700
27,800
14,700
26,800
1.29
1.40
1.30
1.41
1.36
1.35
1.31
1.35
1.23
1.30
1.34
1.25
0.063
0.077
0.108
0.108
0.068
0.122
1.91 E-04
1.88 E-04
2.31 E-05
2.47 E-05
1.77 E-04
1.68 E-05
0.65
0.64
0.85
0.84
0.65
0.89
S-PES 0.9(12 h)
S-PES 0.9(36 h)
S-PES 0.9(60 h)
S-PES 1.2(12 h)
S-PES 1.2(60 h)
13,850
22,700
23,600
9,850
9,150
12,850
20,900
21,400
10,950
21,450
16,050
15,100
8,100
23,300
15,950
(36 h) and S-PES 0.9(60 h). The slope a of the MHS plot for
PES-OH and PES 0.9(12 h) samples is quite similar (a ¼
0.65). The slope a of the MHS plot for S-PES 0.9(36 h) and
S-PES 0.9(60 h) samples is completely different (a ¼ 0.85). In
other words, the S-PES 0.9(36 h) and S-PES 0.9(60 h) samples
were more extended (stiffer chain) in consequence of more
sulfates functional groups in the polymeric chains. This confor-
mation (polyelectrolytes) difference is evidently important
because influence notably the final properties of the polymers.
three disulfonated PES copolymers: S-PES1.2(12 h), S-PES
0.9(12 h) and S-PES1.05(12 h). Each sample was dried at
180 C for 12 h before TGA analysis. The TGA curves in Fig-
ꢀ
ure 9 show that all S-PES copolymers present three thermal
ꢀ
degradation steꢀps ranging from 350 to 800 C, leaving a char
residue at 800 C that increase as raise the DS of copolymers.
The first decomposition step around 380–420 ꢀC could be
ascribed to the decomposition of the ASO3H groups. Second
and third thermal degradation steps occurring in tempera-
ture ranges 450–600 ꢀC and 650–750 ꢀC should be due to
polymer main chain, in accord to the literature.46 TGA curves
of no predried sulfonated samples show also a weight loss
between 100 and 200 ꢀC due to the desorption of water
bonded to the sulfonic groups.
Fourier Transform Infrared Spectroscopy
FTIR spectra were also used to confirm the presence of
pendant ASO3H groups on the sulfonated copolymer chains.
Figure 8 shows the spectra of the parent telechelic PES-OH
(4 K) and of the sulfonated copolymers S-PES0.9(12 h) and S-
PES1.2(12 h) synthesized from it. The FTIR spectra of sulfo-
nated S-PES samples show characteristic absorption bands at
1030 cmꢂ1 due to the symmetric stretching vibrations of
ASO3H groups (indicated with the symbol F in Fig. 8). The
asymmetrical stretching vibrations of ASO3H groups appear at
The glass transition temperatures (Tg) of all sulfonated
copolymers were determined by DMA through heating and
cooling cycles discussed in experimental section, and the
results were listed in Table 1. For comparison, in Table 1
were also reported the Tg data of both telechelic PES-OH
prepolymers. It may be noted that for the S-PES copolymers
obtained at the same reaction time (12 h), the Tg values
increase as increase either the DS and also the degree of po-
lymerization (Xn) of the PES-OH parent, from which were
synthesized. The Tg values of the copolymers prepared by
postcondensation reaction starting from S-PES0.9(12 h) (Fig.
10) and S-PES1.2(12 h) (Fig. 11) increase compared to the
initial copolymers, suggesting that post condensation influ-
enced significantly the molar mass distribution. For both
series of S-PES copolymers we observe that there is a clear
about 1180 cmꢂ1 45
but we could not readily observe it due
,
near overlapping absorbances. All S-SPE copolymers have gave
FTIR spectra similar to those in Figure 8(b,c).
Thermal Properties
Thermal stabilities of the S-PES copolymers were investi-
gated by TGA under nitrogen flow, and in Figure 9, we
report the degradation curves of an unsolfonated PES and of
FIGURE 6 Comparison of the differential MWD of PES-OH(4
K) (a), S-PES-0.9(12 h) (b), S-PES-0.9(36 h) (c) and S-PES-
0.9(60 h) (d).
FIGURE 7 Comparison of MHS plot of PES-OH(4 K) (a), S-PES-
0.9(12 h) (b), S-PES-0.9(36 h) (c), and S-PES-0.9(60 h) (d).
SULFONATED COPOLYETHERSULFONES, SAMPERI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 10 DMA curves of S-PES-0.9(12 h), S-PES-0.9(36 h),
and S-PES-0.9(60 h).
threshold value which is commonly shown by this class of
copolymers with a DS of 10–11 mol %.
CONCLUSIONS
Herein, we have highlight that the synthesis of sulfonated
poly(arylene ether-sulfone) copolymers can be followed com-
bining MALDI-TOF MS and 1H NMR analyses, since detailed
information about the end chains and of the average number
of disulfonated units along the chains can be determined.
SEC-Viscometry analyses allow to reliable data on the molar
mass distribution of these copolymers and DMA data reveal
that a Tg of 244–246 ꢀC were reached for copolymer with a
degree of sulfonation of 10–11 mol %, as increase their molar
mass distribution. Therefore, as the chemical composition, the
end chains, the degree of sulfonation and the molecular mass
distribution affects the properties (i.e., water uptake, in-plane
FIGURE 8 FTIR spectra of PES-OH(4 K) (a), S-PES-1.2(12 h) (b),
S-PES-0.9(12 h) (c).
trend for the glass transition temperature which increase
with the postcondensation time. The Tg increase is the
results of the increase in molar mass which is shown in
Table 1 and from SEC-viscometry data in Table 4. The data
in Table 1 suggest that Tg between 244 and 246 ꢀC is the
FIGURE 9 TGA curves of PES-OH(7 K) (a), S-PES-1.2(12 h) (b),
S-PES-0.9(12 h) (c), and S-PES- 1.05(12 h) (d).
FIGURE 11 DMA curves of S-PES-1.2(12 h), S-PES-1.2(36 h),
and S-PES-1.2(60 h).
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ARTICLE
23 Li, Y.; Wang, F.; Yang, J.; Liu, D.; Roy, A.; Case, S.; Lesko,
proton conductivity, mechanical) and morphology of the pro-
ton exchange membranes based on S-PES copolymers, in the
next work we will investigate these correlations. So based on
the results will be obtained we propose to synthesize and
characterize S-SPES copolymers with higher MMD and DS.
J.; McGrath, J. E. Polymer 2006, 47, 4210–4217.
24 Lee, M.; Park, J. K.; Lee, H. S.; Lane, O.; Moore, R. B.;
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25 Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.;
McGrath, J. E. J Membr Sci 2002, 197, 231–242.
The authors acknowledge the partial financial support from
National Council of Research (CNR) (Rome) and from Italian
Ministry for University and Research (MIUR) (PRIN 2007).
26 Schuster, M.; Kreuer, K. D.; Andersen, H. T.; Maier, J. Mac-
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27 Iojoiu, C.; Chabert, F.; Marechal, M.; Kissi, N.; El Guindet, J.;
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